Existing E-Mount lenses will fit on the A7 and A7R, but will project a smaller image onto their 'Full Frame' sensor. The cameras will recognize that the lenses are not 'Full Frame' lenses, and will capture a smaller image using only the center portion of the sensor. The 24MP A7 will capture only a 10MP image, and the 36MP A7R will capture only a 15MP image.

This is similar to what the 'Full Frame' Sony A900, A850 and A99 do when an 'APS-C only' DT lens is mounted on them.